The present invention relates to optical disk drives that use multiple beams to simultaneously read or write multiple tracks of an optical disk. More specifically, the present invention provides a multi-beam optical system that detects and corrects for magnification errors and variations in track pitch while simultaneously reading or writing multiple tracks of an optical disk.
Due to their high storage density, long data retention life, and relatively low cost, optical disks are becoming increasingly popular as a means to distribute information. Large format disks have been developed for storing full length motion pictures. The compact disk (CD) format was developed and marketed for the distribution of musical recordings and has replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM and Digital Versatile Disk (DVD), have become prevalent in the personal computer field, and the DVD format may soon replace videotape as the distribution medium of choice for video information.
Recently, relatively inexpensive optical disk writers and writable optical media have become available, making optical disks popular as backup and archival storage devices for personal computers. The large storage capacity of writable optical disks also makes them ideal for use in multimedia authoring and in other applications which require access to large amounts of storage. Current writable optical disk technologies include several write-once technologies, such as CD-Recordable (CD-R). A few technologies permit writing, erasing, and rewriting data on a disk, such as Mini-Disk (MD), which uses magneto-optical technology. Other writable formats employ phase-change and dye-polymer technology. Recent advances in writable optical disk technology have made rewritable optical media more practical, and the specification for DVD-RAM calls for use of high-capacity rewritable optical media.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, is encoded as a series of pits in a reflective surface within the disk. The pits are arranged along a spiral or circular track. Data is read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk. By rotating the optical disk, the light reflected from the surface of the disk is modulated by the pattern of the pits rotating into and out of the laser""s field of illumination. Optical and imaging systems detect the modulated, reflected, laser light and produce an electrical signal which may be decoded to recover the digital data stored on the optical disk.
Data is typically recorded on writable optical disks by using a higher power laser than is used for reading. The media for use with optical disk writers typically includes a recording layer, made of a material that changes its optical characteristics in response to the presence of the beam from the high power laser. The high power laser is used to create xe2x80x9cpitsxe2x80x9d in the recording layer which have a different reflectivity than surrounding areas of the disk, and which can be read using a lower power reading beam. In systems having the ability to erase and re-record data, a laser having a power output between the low power used for reading and the high power used for writing may be used to erase data. Alternatively, some systems employ a laser which outputs a different wavelength of light to erase data from the optical media. The methods used to write and erase optical disks depend on the type of recordable media being used.
To be able to write or retrieve data from an optical disk, the optical systems include an optical head which may be positioned to read or write data on any disk track. Processor-driven servo mechanisms are provided for focusing the optical system and for keeping the optical head positioned over the track, despite disk warpage or eccentricity.
Because in most previously known systems the data is read or written serially, i.e. one bit at a time, the maximum data transfer rate for an optical disk reader or writer is typically determined by the rate at which the pits on the disk (or the correct positions for pits to be written) pass by the optical head. The linear density of the bits and the track pitch (distance between tracks) are fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.5 xcexcm (xc2x10.1 xcexcm), while DVD employs a track pitch only about one-half as wide.
Previously known methods of increasing the data transfer rate of optical disk readers and writers have focused on increasing the rate at which the pits pass by the optical head by increasing the rotational speed of the disk itself. Currently, drives with rotational speeds of up to 16xc3x97 standard speed are commercially available, and even faster reading speeds have been achieved by moving to constant angular velocity designs. Higher disk rotational speeds, however, place increased demands on the optical and mechanical subsystems within the optical disk player, create greater vibration, and may make such players more difficult and expensive to design and manufacture. Higher rotation speeds also make accurately writing data to a disk more difficult, so few CD-R systems are available that record at faster than 4xc3x97 standard speed.
A cost effective alternative to increasing the disk rotational speed to provide faster optical disk drives is to read or write multiple data tracks simultaneously. If, for example, seven tracks could be read or written simultaneously, an optical disk drive which rotates the disk at 8xc3x97 standard speed would provide performance equivalent to a 56xc3x97 optical disk drive.
Similar techniques have been employed to provide high-speed optical disk readers. Numerous methods for generating multiple beams to read several tracks simultaneously have been used. U.S. Pat. No. 4,459,690, to Corsover, for example uses acousto-optical techniques to split a beam into multiple beams for use in reading an optical disk. Other systems have used a diffraction grating to generate multiple beams used to simultaneously illuminate multiple tracks. The system described in commonly assigned U.S. Pat. No. 5,426,623, to Alon et al., uses a wide area illumination beam, which illuminates multiple tracks at once to simultaneously read multiple tracks of an optical disk.
It should be noted that as used herein, a data track is a portion of the spiral data track of a typical optical disk, and follows the spiral for one rotation of the disk. Thus, a drive capable of reading multiple data tracks simultaneously will read multiple portions of the spiral data track at once. For optical disks having concentric circular tracks, a data track would refer to one such circular track. For disks having multiple concentric spiral tracks, such as those described in commonly assigned, copending U.S. patent application Ser. No. 08/885,425, filed Jun. 30, 1997, a data track would refer to one of the concentric spiral tracks.
Designing an optical disk drive that simultaneously writes multiple tracks of an optical disk presents slightly different challenges than designing a system which only reads multiple tracks simultaneously. First, each of the beams used to write to the disk must be able to be separately modulated, to record different data on each of the tracks. Consequently, designs like those described above, that split a single beam or employ a wide area beam, will not generally work for a drive that can both read and write. Instead, multi-beam optical disk drives which can write multiple tracks simultaneously use multiple laser diodes, which can be individually modulated, to generate the beams used for writing. Such an array of laser diodes is described, for example, in U.S. Pat. No. 5,144,616 to Yasukawa et al.
Additionally, since most optical disk formats arrange their data along a single long spiral, there may be difficulties with data alignment and timing when writing multiple tracks simultaneously. Insuring alignment of the data being written by multiple beams on different parts of the same spiral may be very difficult. These problems are overcome in some writable formats by using a pre-formatted disk, on which the tracks are already laid out, and the disk already contains clocking and timecode information before data is written to the disk. CD-Recordable (CD-R), which is currently the most popular writable optical disk format uses such a scheme, as do DVD-R and DVD-RAM, which will probably replace CD-R over the next few years. Other formats may include header information that describes the track pitch.
Although using a format such as CD-R, DVD-R, or DVD-RAM, in which the tracks are pre-formatted, solves the most difficult data alignment and timing problems faced by a multi-beam optical disk drive, it creates a new problem with keeping the beams aligned with the tracks while writing. Specifically, since the tracks are pre-arranged on the disk, it is necessary to insure that each of the multiple beams aligns with one of the tracks during writing. This same alignment of the beams with the tracks also must be maintained while reading from the disk.
Manufacturing tolerances may lead to minor differences in magnification of an optical head, leading to minor differences in the spacing of the beams between systems. Additionally, there is some variation in the track pitch allowed in the specifications for commonly used optical disk formats, such as CD-ROM, CD-R, DVD, DVD-R and DVD-RAM formats. A multi-beam optical disk drive must be able to detect and correct for these magnification errors and track pitch variations to insure that the beams used to read from and write to the disk are properly aligned with the tracks.
It would therefore be desirable to provide a multi-beam optical head, and methods of use, that enable detection and correction of magnification and track-pitch errors while simultaneously reading or writing multiple tracks of data from or to an optical disk. The capability to correct for such errors would provide improved alignment of the beams with the tracks being read or written on the disk, and make simultaneous reading or writing of multiple tracks practicable.
It further would be desirable to provide a multi-beam optical head, and methods of use, that enable detection of a track pitch of an optical disk, and that employ the detected track pitch to correct magnification and track-pitch errors while simultaneously reading or writing multiple tracks of an optical disk.
In view of the foregoing, it is an object of the present invention to provide methods and apparatus for detecting and correcting misalignments between the beams of a multi-beam optical disk drive and the tracks of an optical disk caused by magnification error and track-pitch variation.
It is a further object of this invention to provide methods and apparatus that enable detection of a track pitch of an optical disk, and that employ the detected track pitch to correct magnification and track-pitch errors while simultaneously reading or writing multiple tracks of an optical disk.
In accordance with the principles of the present invention, this is accomplished by detecting a magnification error, and then using that information to vary the optical power or magnification of the optical system. Accordingly, the reading or writing beams of the system may be focused onto the disk to adjust the spacing between the beams, so they are aligned with the tracks on the optical disk. A number of methods and apparatus employing the principles of the present invention are provided.
A first embodiment of a magnification correction system built in accordance with the principles of the present invention uses a movable lens arrangement to adjust the magnification of the system. An alternative embodiment uses one or more prisms to build an anamorphic variable power optical system that adjusts the spacing of the beams by rotating a prism. Another alternative embodiment uses a lens with a curvature that varies along its length to correct magnification errors. In certain embodiments, the optical head may be moved laterally with respect to the tracks to compensate for the magnification and track pitch variation effects.
Magnification errors may also be corrected by changing the effective distance between the laser diodes and, optionaly, or the spacing between photodetectors used to image the multiple data tracks. This is done by adjusting the position of the array of laser diodes and photodetector elements, relative to the radial direction of the optical disk, so that the beams align with the tracks of the optical disk.
Methods are also described for detecting magnification and track pitch errors, so that those errors may be taken into account when simultaneously reading or writing several tracks of an optical disk. In one embodiment, the track pitch is determined using track pitch information recorded on the disk; in other embodiments, an initial calibration step may be employed wherein the track pitch is computed by a track counting method or by analyzing the jitter rate obtained in reading block header (or other pre-recorded) data from the disk.
Any of the foregoing methods may be used to provide continuous correction of magnification errors, thereby enabling the beams of a multi-beam optical disk drive to remain aligned with the tracks of an optical disk.